The Exhumation of Eclogite-Facies Metamorphic Rocks—a Review of Models Confronted with Examples from the Alps

Abstract

We summarize recently proposed models for the exhumation of (ultra-) high pressure ([U]HP) metamorphic units within collisional orogens. In terms of mechanics, exhumation is either caused by buoyancy or by externally applied stresses. In terms of kinematics, exhumation is either by erosion, extension, or extrusion. The tectonic scenarios that may lead to the exhumation of (U)HP units are: (1) flow within a low-angle or high-angle corner; (2) entrance of the continental margin into the subduction zone; (3) cessation of subduction; (4) slab breakoff; (5) extension in the backarc; and (6) serpentinization of the mantle wedge. The applicability of these models are discussed for some (U)HP terrains in the Alps, in particular the Koralm Complex of the Austroalpine Unit, and the Internal Penninic Nappes of the Eastern, Central, and Western Alps, (in particular, the Eclogite Zone, the Adula Nappe, and the Monte Rosa Nappe). Within the Austroalpine Koralm Complex of the Eastern Alps, the exhumation of HP rocks seems to be related to the development of a major detachment within the lower crust (Plattengneis shear zone), subsequent to the formation of an orogenic wedge. The central parts of the Koralm Complex were exhumed to a larger extent than the northern, and in particular the southern parts, as indicated by a continuous decrease of pressures toward north and south. Thus, we suggest that the Plattengneis shear zone was not related to the compressional phase during nappe stacking, but to the extensional exhumation of the Middle Austroalpine basement complexes that were previously affected by eclogite facies metamorphism. The tectonometamorphic evolution of (U)HP Internal Penninic Nappes, which are assumed to have originated from the European continental margin, may be characterized by(buoyancy-driven) extrusion of eclogite facies crustal wedges from the subduction channel with a low-angle corner geometry. During the early phase of exhumation, the pressure-temperature path is characterized by cooling in the Dora Maira Massif, the Gran Paradiso, the Monte Rosa Nappe, and the Eclogite Zone; this argues for exhumation within a still-active subduction zone. In the Adula Nappe, decompression was isothermal. At amphibolite- to greenschist facies conditions, the exhumation mechanism of these units changed to crustal extension, accompanied by minor heating during decompression (except for the Adula Nappe). This argues for cessation of subduction and removal of the cooling effect.